When a strong coherent light such as a laser light passes through a medium having a secondary non-linear optical effect (or a wavelength conversion element, hereinafter referred to as a crystal because such a medium is often an optical crystal), such crystal outputs a light wave having a frequency double the frequency of the input light. Such a crystal also outputs a light wave having a frequency which is the sum or difference of the frequencies of two input components if the input light includes a plurality of frequency components. These are referred to as the generation of a second harmonic, generation of a sum frequency, and generation of a difference frequency, respectively. To enhance the conversion efficiency of a crystal, or the ratio of the converted output light intensity of a new frequency to the input light intensity, it is necessary to satisfy a condition called phase match.
The condition for phase match requires that the refractive indexes of a crystal for a fundamental wave (input light) and a generated second harmonic (output light) are equal in the case of the generation of the second harmonic. Such phase match can be accomplished by utilizing the birefringence of a crystal, for example, by using an ordinary ray as the fundamental wave (input light) and an extraordinary ray as the second harmonic (output light). Depending on the crystal type, by appropriately selecting the orientation and temperature of the crystal, this phase match condition can be achieved within a predetermined wavelength range of the input light. Particularly, a crystal which accomplishes the phase match condition by changing crystal orientation is called an angle phase match type crystal.
As an example of an angle phase match type crystal, there is beta-barium borate (.beta.-BaB.sub.2 O.sub.4) (hereinafter referred to as BBO). BBO has several excellent features as a wavelength conversion element. It is a uniaxial crystal that has a large non-linear susceptibility to the ultraviolet region, a wide transparency region, phase match in a wide wavelength range, chemical stability, and a relatively high optical destruction strength. It has been proposed in the past to use such BBO as a wavelength conversion element in combination with an optical cavity having a plurality of reflecting surfaces for converting laser light (H. Souma, T. Sato, T. Nishimata, I. Shindo: Proceeding of International Quantum Electronics Conference 1988 Tokyo, PD-7 (1988) 19-22).
If the intensity of the input light is low, the intensity of the output light of the wavelength converter is proportional to the square of the intensity of the input light. If a low-intensity light from a continuous-wave laser is used as the input light, the conversion efficiency is extremely low. Accordingly, for high conversion efficiency, it is necessary to use an optical focusing system in combination with an optical cavity to focus the fundamental wave (input light) onto the crystal. However, since the intensity of the light within the optical cavity is much larger than that of the output light which is emitted outside of the cavity, the crystal is readily destroyed or damaged if the light within the cavity is focused to one point on the crystal by an optical focusing system having spherical lenses.
Thus, in previously known systems, the practical conversion efficiency of an apparatus for wavelength conversion of laser light is determined by the optical destruction or damage condition of the crystal.
With respect to focusing light, it has been proposed that light be focused on a non-linear optical crystal (such as BBO), which is a wavelength conversion element, by using cylindrical lenses (T. G. M. Freegarde, J. Courts, A Corney, "Second Harmonic Generation in Beta-Barium Borate using Elliptical Focusing, "Non-linear Optical Properties of Materials, 1988 Technical Digest Series Volume 9, Aug. 22-25, 1988, Troy, N.Y., pp. 77-80" and V. D. Volosov, E. V. Nilov, "Effect of the Spatial Structure of a Laser Beam on the Second Harmonic in ADP and KDP Crystals," Optical Spectroscopy (USSR) Vol. 21 (1966), pp. 392-394). However, in these references, placing a wavelength conversion element in an optical cavity was not considered, and thus light was not strongly amplified and the optical destruction of the wavelength conversion element was not a significant problem. In addition, in these references, there was no suggestion with respect to the enhancement of the wavelength conversion efficiency of continuous-wave laser lights of relatively low outputs.